In the school hallway the wall next to Room 12 changes from a fire hose closet to a principal's office sign between scenes.
When Opie is standing in the school's hallway with a couple of girls, there is a sign on one of the walls that reads "Principals Office." It is misspelled. The word "Principals" should have an apostrophe to make it a possessive (as in "Principal's").
When Arnold and Opie are standing next to the mirror, in the mirror it shows a lamp next to the painting, when it shows Andy sitting on the couch, the lamp has suddenly disappeared.
Aunt Bee tells the about-to-turn 13 Arnold Bailey that she remembers receiving his birth announcement. However, Opie, who is the same age as Arnold, was already six years old when Aunt Bee came from West Virginia to live with the Taylors. Arnold would also have been five or six years old when Aunt Bee came to Mayberry, long after birth announcements would have been sent.